<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Chapter 19.  The Logging Subsystem</title>
    <link rel="stylesheet" href="gettingStarted.css" type="text/css" />
    <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
    <link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
    <link rel="up" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
    <link rel="prev" href="lock_nondb.html" title="Locking and non-Berkeley DB applications" />
    <link rel="next" href="log_config.html" title="Configuring logging" />
  </head>
  <body>
    <div xmlns="" class="navheader">
      <div class="libver">
        <p>Library Version 12.1.6.2</p>
      </div>
      <table width="100%" summary="Navigation header">
        <tr>
          <th colspan="3" align="center">Chapter 19.  The Logging Subsystem </th>
        </tr>
        <tr>
          <td width="20%" align="left"><a accesskey="p" href="lock_nondb.html">Prev</a> </td>
          <th width="60%" align="center"> </th>
          <td width="20%" align="right"> <a accesskey="n" href="log_config.html">Next</a></td>
        </tr>
      </table>
      <hr />
    </div>
    <div class="chapter" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="log"></a>Chapter 19.  The Logging Subsystem </h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="log.html#log_intro">Introduction to the logging
        subsystem</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="log_config.html">Configuring logging</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="log_limits.html">Log file limits</a>
            </span>
          </dt>
        </dl>
      </div>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="log_intro"></a>Introduction to the logging
        subsystem</h2>
            </div>
          </div>
        </div>
        <p>
        The Logging subsystem is the logging facility used by
        Berkeley DB. It is largely Berkeley DB-specific, although it
        is potentially useful outside of the Berkeley DB package for
        applications wanting write-ahead logging support. Applications
        wanting to use the log for purposes other than logging file
        modifications based on a set of open file descriptors will
        almost certainly need to make source code modifications to the
        Berkeley DB code base.
    </p>
        <p>
        A log can be shared by any number of threads of control. The
        <a href="../api_reference/C/envopen.html" class="olink">DB_ENV-&gt;open()</a> method is used to open a log. When the log is no
        longer in use, it should be closed using the <a href="../api_reference/C/envclose.html" class="olink">DB_ENV-&gt;close()</a>
        method.
    </p>
        <p>
        Individual log entries are identified by log sequence
        numbers. Log sequence numbers are stored in an opaque object,
        an <a href="../api_reference/C/lsn.html" class="olink">DB_LSN</a>.
    </p>
        <p>
        The <a href="../api_reference/C/logcursor.html" class="olink">DB_ENV-&gt;log_cursor()</a> method is used to allocate a log cursor. Log
        cursors have two methods: <a href="../api_reference/C/logcget.html" class="olink">DB_LOGC-&gt;get()</a> method to retrieve log
        records from the log, and <a href="../api_reference/C/logcclose.html" class="olink">DB_LOGC-&gt;close()</a> method to destroy the
        cursor.
    </p>
        <p>
        There are additional methods for integrating the log
        subsystem with a transaction processing system:
    </p>
        <div class="variablelist">
          <dl>
            <dt>
              <span class="term">
                <a href="../api_reference/C/logflush.html" class="olink">DB_ENV-&gt;log_flush()</a>
              </span>
            </dt>
            <dd>
                Flushes the log up to a particular log sequence
                number.
            </dd>
            <dt>
              <span class="term">
                <a href="../api_reference/C/logcompare.html" class="olink">DB_ENV-&gt;log_compare()</a>
              </span>
            </dt>
            <dd>
                Allows applications to compare any two log
                sequence numbers.
            </dd>
            <dt>
              <span class="term">
                <a href="../api_reference/C/logfile.html" class="olink">DB_ENV-&gt;log_file()</a>
              </span>
            </dt>
            <dd>
                Maps a log sequence number to the specific log
                file that contains it.
            </dd>
            <dt>
              <span class="term">
                <a href="../api_reference/C/logarchive.html" class="olink">DB_ENV-&gt;log_archive()</a>
              </span>
            </dt>
            <dd>
                Returns various sets of log filenames. These
                methods are used for database administration; for
                example, to determine if log files may safely be
                removed from the system.
            </dd>
            <dt>
              <span class="term">
                <a href="../api_reference/C/logstat.html" class="olink">DB_ENV-&gt;log_stat()</a>
              </span>
            </dt>
            <dd>
                The display <a href="../api_reference/C/db_stat.html" class="olink">db_stat</a> utility used the <a href="../api_reference/C/logstat.html" class="olink">DB_ENV-&gt;log_stat()</a> method
                to display statistics about the log.
            </dd>
            <dt>
              <span class="term">
                <a href="../api_reference/C/envremove.html" class="olink">DB_ENV-&gt;remove()</a>
              </span>
            </dt>
            <dd>
                The log meta-information (but not the log files
                themselves) may be removed using the <a href="../api_reference/C/envremove.html" class="olink">DB_ENV-&gt;remove()</a>
                method.
            </dd>
          </dl>
        </div>
        <p>
        For more information on the logging subsystem methods, see
        the <a href="../api_reference/C/lsn.html#loglist" class="olink">Logging
        Subsystem and Related Methods</a> section in the
        <em class="citetitle">Berkeley DB C API Reference Guide.</em>
    </p>
      </div>
    </div>
    <div class="navfooter">
      <hr />
      <table width="100%" summary="Navigation footer">
        <tr>
          <td width="40%" align="left"><a accesskey="p" href="lock_nondb.html">Prev</a> </td>
          <td width="20%" align="center"> </td>
          <td width="40%" align="right"> <a accesskey="n" href="log_config.html">Next</a></td>
        </tr>
        <tr>
          <td width="40%" align="left" valign="top">Locking and non-Berkeley DB
        applications </td>
          <td width="20%" align="center">
            <a accesskey="h" href="index.html">Home</a>
          </td>
          <td width="40%" align="right" valign="top"> Configuring logging</td>
        </tr>
      </table>
    </div>
  </body>
</html>
